Originally Posted by HorseGuru
He is 15 and I have noticed when he does it his ears are not pinned back like he is angry or anything he is just having a good time and I ride them through now I just make sure his head is up and when I see him start it get excited I make sure his head is up
Sounds like he is just feeling good. Assuming you are in a safe place to do so, I would make him move forward and WORK (be that a faster trot, moving laterally, etc). He will hopefully realize its a lot nicer to just trot along than it is to do a long trot or half pass for miles down the trail.